CHM 7292-Advanced Organic Chemistry
Villanova University
Dr. Joseph W. Bausch
Exam 1, in-class part (covering chapters 1-3)
September 30, 2004
Please legibly write answers in the blue booklet.
1.
(9)
Rationalize the following A-values (i.e., equatorial preferences):
CH3
CH3
A value (-G°) = 1.8 kcal/mol
CH3
O
O
O
O
CH3
A value (-G°) = 4.0 kcal/mol
CH3
S
S
S
S
CH3
A value (-G°) = 1.8 kcal/mol
2.
(6) Draw Lewis structures for both dimethylsulfoxide, CH3S(O)CH3, and
dimethyl nitroxide, CH3N(O)CH3. The atom-atom connectivity is shown
below for each. Evaluate each structure in terms of its contribution to the
overall resonance hybrid:
H
O
S
H
H
C
C
H
H
H H
3.
O
N
H
C
C
H
H
H H
(5) Our book states in chapter 2: “Any structure is chiral if it does not have
(blank), that is, if it belongs to point group Cn or Dn. The “blank” is a
symmetry operation. Which symmetry operation is it?
1
4.
(8) Note that the following represents a change in configuration, not
conformation. Remembering that each 1,3-diaxial interaction involving a
methyl and a hydrogen (a classic gauche butane interaction) we estimate as
being worth 0.9 kcal/mol, estimate how large (in kcal/mol) a 1,3-diaxial
interaction between two methyl groups is based upon the following
experimental data that indicates the cis isomer is ~3.7 kcal/mol more stable
than the trans:
CH3
CH3
CH3
Pd
H3 C
CH3
CH3
H3 C
H3 C
G° ~ -3.7 kcal/mol
5.
(4)
Assign as R or S each chirality center in the following molecules:
CO2H
H
OH
HO2C
H
CH2CO2H
6.
CO2H
HO
H
H
CO2H
CH2CO2H
(5) Shown below, believe it or not, is the structure originally proposed for
cordecypic acid, []D = +40.3°, which has since been shown to be incorrect.
Suggest a reason to be skeptical about the original structure, which is given
below (hint: look carefully for a symmetry element!).
CO2H
HO
OH
HO
OH
7.
(5) Cyclobutane (C4H8) prefers a slightly puckered conformation instead
of one where the ring is planar. What does this say about the relative
importance of torsional strain versus angle strain in this system? Attempt to
illustrate your answer with drawings.
2
8.
(8) In each of the following structures, indicate whether the circled groups
are:
i.
ii.
iii.
iv.
a.
homotopic or heterotopic
if heterotopic, whether structurally heterotopic or stereoheterotopic.
If stereoheterotopic, whether they are enantiotopic or diastereotopic.
If enantiotopic or diastereotopic, indicate which is pro-R and which is
pro-S.
CH3
Cl
H3C CH3
b.
CH3
c.
H
H
3
CO2H
HO
H
H
OH
H
H
CH2OH